Abstract
67Ga-citrate is used for the clinical scanning of the malignant tumor, but the mechanism of its affinity for the tumor is unknown. In order to investigate the mechanism of the accumulation of 67Ga in the tumor, experiments were carried out as follows.
Five chemical forms of 67Ga-citrate (carrier-free) , 67Ga-citrate (containing carrier Gacitrate) , 67Ga-nitrate (carrier-free) , 67Ga-nitrate (containing carrier Ga-nitrate) and 67Ga-EDTA (carrier-free) were prepared. Their affinities for the malignant tumor were examined. These five preparations were injected intravenously to the rats which had subcutaneously transplanted Yoshida Sarcoma and these rats were sacrificed 3 hours, 24 hours and 48 hours after the injection. The radioactivities of the tumor, blood, muscle, liver, kidney and spleen were measured by a well-type scintillation counter. The retention values (% dose per g tissue weight) in the tumor, blood, muscle, liver, kidney and spleen were calculated.
67Ga-citrate (carrier-free) and 67Ga-nitrate (carrier-free) have very close affinities to the malignant tumor and their retention values in the tumor were approximately constant during 48 hours. Their excretion into urine was slower than other three preparations. The affinities of 67Ga-citrate (containing carrier Ga-citrate) and 67Ga-nitrate (containing carrier Ga-nitrate) for the tumor were weak and the excretion into urine was faster than those of carrier free. 67Ga-EDTA was rapidly excreted into urine and its affinity for the tumor was weak. Besides, the experiment with rats which had intraperitoneally transplanted Yoshida Sarcoma showed that 67Ga-citrate (carrier-free) and 67Ga-nitrate (carrier-free) were taken up considerably by Yoshida Sarcoma ascites cells.
From these experimental results, it may be deduced that a suitable chemical form of 67Ga has strong affinity for tumors. 67Ga-nitrate becomes gallium-ion in water, and it is known that 67Ga-citrate becomes gallium-ion when it was diluted with the adequate amount of water. Therefore the chemical form suitable for the selective uptake by tumor cells should be carrier free and able to become gallium-ions easily in the body.
Chemical properties of gallium were also studied. The binding capacity of gallium-ion to protein is not strong, but its binding capacity to glass is very strong. It is likely that the gallium-ion is either physiologically necessary for cancerous cells, or, if not, it has specifically strong affinity for tumors for unknown reasons.
